My dad had this stud finder tool used to find studs in a wall when there is already drywall on. My question is any one have experience with them? The one I have (previously my dads) is black and orange I think. On the left is the button you hold while you slowly drag the machine across the wall. Then it has these lights. 4 green and 1 red I think. or the other way around. I can't figure out which one means the "edge" and the center of each stud. Also, can this be used upside down too? I want to hang my punching bag in the garage, and will attached it to the joists (is that what they are called, couldn't think of anything else) that run along the house. Comments? Pictures and specs on the machine tomorrow.